Recently, compounds have been discovered which, in certain phases, become superconducting at or above 90.degree. K. This immediately makes these compounds, labelled "high T.sub.c ", much more practical for use, since previous superconductors required cooling by liquid helium (.about.4.degree. K.). The high T.sub.c materials can be cooled to their critical temperature by liquid nitrogen (.about.77.degree. K.), which is easier to handle and considerably less costly.
Several uses of superconducting electronic devices depend on Josephson weak links. In such devices, if two superconductors are connected together through a weak link, such as a thin oxide barrier current can flow by tunneling through the barrier without any voltage. Josephson-effect weak links are used as SQUIDs (superconducting quantum interference devices) and for digital switches. While SQUID magnetometers are well known at lower critical temperatures (T.sub.c), on the order of 4.degree. K., difficulties have been encountered in reliably forming SQUIDs and other Josephson-effect based devices using the high T.sub.c materials developed in recent years. Nonetheless, it would be desirable to apply high T.sub.c materials to these devices to make the devices more practical, enabling relatively widespread use.
A considerable amount of work has been devoted to the development of high T.sub.c materials which possess sufficient reliability to permit their incorporation into electronic devices. Early work in high T.sub.c copper oxides, including YBa.sub.2 Cu.sub.3 O.sub.x (YBCO) films, indicated that high temperature (850.degree. K.) annealing was required to stabilize the oxygen content within the sample. The requirement of a high-temperature treatment would introduce considerable difficulty in fabrication of superconducting interconnects.
Another drawback of many of the high T.sub.c superconductors is that they have very short coherence lengths, on the order of nanometers, which places them beyond the capabilities of current. photolithographic techniques. Therefore, other means for shaping and activating the junctions must be determined.
It has been shown that prolonged exposure of YBa.sub.2 Cu.sub.3 O.sub.x films to visible light enhances the compound's superconductive properties (see, e.g., V. I. Kudinov, et al., "Dependence of Photo-Induced Conductivity on Illumination Dose in YBa.sub.2 Cu.sub.3 O.sub.6.4 Films", Phys. Lett. A, V 157, N. 4.5, pp. 290-294). The photoexcitation of YBa.sub.2 Cu.sub.3 O.sub.x (YBCO) reduces the resistivity of the material, and the effect persists for long times, on the order of days.
It would be desirable to incorporate the photoexcitability of high T.sub.c films for the purpose of fabricating reproducible and reliable Josephson weak links. It is to such a purpose that the present invention is directed.